Sunan Ibn Majah, Hadith 4086 ⓘ
It was narrated that Sa'eed bin Musayyab said: "We were with Umm Salamah and we were discussing Mahdi. She said: 'I heard the Messenger of Allah say: "Mahdi will be one of the descendents of Fatimah."
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَحْمَدُ بْنُ عَبْدِ الْمَلِكِ، حَدَّثَنَا أَبُو الْمَلِيحِ الرَّقِّيُّ، عَنْ زِيَادِ بْنِ بَيَانٍ، عَنْ عَلِيِّ بْنِ نُفَيْلٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، قَالَ كُنَّا عِنْدَ أُمِّ سَلَمَةَ فَتَذَاكَرْنَا الْمَهْدِيَّ فَقَالَتْ سَمِعْتُ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ يَقُولُ " الْمَهْدِيُّ مِنْ وَلَدِ فَاطِمَةَ " .
also referenced under: Book 36, Hadith 161 (In-Book) & Volume 5, Book 36, Hadith 4086 (DarusSalam)